WHISTLE BEACON

Published:2011/8/14 21:21:00 Author:Li xiao na From:SeekIC

Model aircraft sometimes have the unpleasant property of choosing their own landing field, and doing so in terrain where they are hard to see. In such cases, the model finder described here can make the search easier.

An unplanned landing is already bad enough, but to make matters worse, it sometimes happens in a cornfield. That often means searching crisscross through the field for several hours and considerable damage to the field, -which neither the farmer nor the insurance company finds especially pleasant. This whistle beacon circuit will at least help you find your model more quickly, by emitting a loud signal if the transmitter is switched off or the joystick is moved past a previously programmed position. The searchers (and unfortunately, the farmer as well) can hardly help hearing this signal. The circuit is designed to avoid sacrificing a receiver channel. The model finder can be inserted between the receiver and the servo as necessary. The only thing that requires attention is to ensure that the connector on the circuit board matches the pin assignment of the servo cable (see Figure 1).



Receiver signal evaluation

Many commercial model finders evaluate the absence of the remote control signal as the criterion for enabling the alarm generator. What they forget is that if the transmitter is switched off. a large number of noise signals are intermittently present at the receiver output, and the downstream electronics can mistake these signals for control signals. The circuit shown in Figure 2 refuses to be misled by such signals, since it compares the received signal with a reference signal and only responds if the pulse width of the received signal is less than a predefined value.



The evaluation circuit primarily consists of a pair of retriggerable mono-stable multivibrators (ICi) whose timing characteristics are determined by external circuitry. IC1a generates a reference pulse at its output (pin 13) in response to a rising edge of the input signal (clock signal from the receiver).

The width of this pulse can be set between approximately 1 ms and 2 ms using trimpot P1. IC1b is triggered on the falling edge of the signal at the output of IC1a if the signal coming from the receiver has a Low level. The switching signal is taken from the output of IC1b (pin 5). If the signal from the receiver drops below the pulse width defined by the user, pin 5 will have a High level, which energizes the following oscillator stage. The oscillator is built using a 555 timer IC. This IC can drive a piezo acoustic transducer (without integrated electronics) without any additional circuitry. The frequency-determining components (R4. R5 and C4) set the frequency of the oscillator to around 4 kHz. C4 is charged by R5 and R6 in series and discharged via R6 alone. The oscillator is enabled when the reset input (pin 4) has a High level. A 220-11 resistor can also be inserted in series with the transducer to reduce the loudness; this will also change the frequency.

Alternatively, a transducer with integrated electronics can be used to reduce the complexity of the circuitry. In this case, a driver transistor and acoustic transducer are connected at point A instead of IC2. The integrated electronics then takes over the task of the oscillator (IC2) and drives the piezoelectric wafer in the transducer at a frequency of around 2-3 kHz. Naturally, this arrangement will not work with a transducer lacking integrated electronics, which cannot be visually distinguished from a type that has its own oscillator.

In the relevant specialist magazines, commercial vendors of model finders try to outdo each other with the loudness of their circuitry. For our models, we find a transducer with a sound pressure level greater than 85 dB(A) to be fully adequate. Naturally, the sound must be able to actually emerge via a hole or a perforated region of the enclosure.

Calibration

The only calibration point in the circuit is the threshold for enabling the acoustic transducer. This can be set using trimpot P1. With the transmitter switched on. PI should be rotated until no tone can be heard at any joystick position. With the transmitter switched off. PI should be rotated until the tone starts to sound. Of course, you can also adjust the model finder to enable the transducer at a certain joystick position.

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